Optical waveguide and optical concentration measuring apparatus

ABSTRACT

Sticking of core layer is suppressed, and deterioration of sensitivity of a sensor is prevented. An optical waveguide ( 10 ) includes a substrate ( 15 ), a core layer ( 11 ), a support, and a protrusion ( 18 ). The core layer ( 11 ) can transmit light. The support connects at least a portion of the substrate ( 15 ) and a portion of the core layer ( 11 ) together. The support supports the core layer ( 11 ). The protrusion ( 18 ) is arranged at a position different from a position of the support in a space between the substrate ( 15 ) and the core layer ( 11 ). The protrusion ( 18 ) has a maximum height at a position deviated from a central position cp of the core layer ( 11 ) in a width direction. The protrusion ( 18 ) protrudes toward the core layer ( 11 ) from the substrate ( 15 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to and the benefit of JapanesePatent Applications No. 2018-163489 and No. 2018-163649 filed on Aug.31, 2018, Japanese Patent Application No. 2019-096823 filed on May 23,2019, and Japanese Patent Application No. 2019-107436 filed on Jun. 7,2019, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to an optical waveguide and an opticalconcentration measuring apparatus.

BACKGROUND

When a refractive index of a material of a structure, such as a thinfilm formed from crystals or the like, is greater than a refractiveindex of a material external to the structure, light transmitted throughthe structure travels repeatedly undergoing total internal reflection atthe boundary between the structure and the outside of the structure.

As illustrated in FIG. 17, when light L transmitted through a structure51 is entirely reflected on the boundary between the structure 51 and asubstance 53, the light L travels through the structure 51 and partiallyextends to the substance 53 that has a small refractive index. Such anextension is referred to as an evanescent wave, which can be absorbed bya substance adjacent to the structure 51 when the light L is transmittedthrough the structure 51. In FIG. 17, a light intensity E1 represents anintensity of the light L transmitted in the structure 51, and a lightintensity E2 represents an intensity of an evanescent wave. This enablesdetection and identification of the substance 53 in contact with thestructure 51 based on a change in the intensity of the light Ltransmitted through the structure 51. An analyzing method using theprinciple of the evanescent wave described above is referred to as anATR (Attenuated Total Reflection) method and used in chemicalcomposition analysis of substances and the like.

Patent literature (PTL) 1 set forth below proposes an optical waveguidesensor in which the ATR method is applied to a sensor. This opticalwaveguide sensor is configured to irradiate light into a core layerformed on a substrate and detect a substance in contact with the corelayer by using the evanescent wave.

Typically, infrared radiation is used as the light to be transmittedthrough the core layer. Because substances have the property ofselectively absorbing infrared radiation of particular wavelengths, asubstance to be measured can be analyzed and sensed by transmittinginfrared radiation in accordance with an absorption spectrum of thesubstance to be measured.

In order to improve sensitivity of a sensor, it is necessary to reducean optical loss of the core layer and increase a region of interactionbetween the substance to be measured and the evanescent wave. As such,for example, a structure in which a portion of a waveguide is floatedwith respect to the substrate by using intermittently arranged supportstructures is proposed.

CITATION LIST Patent Literature

PTL 1: JP2005-300212A

SUMMARY

However, the structure in which a portion of the waveguide is floatedwith respect to the substrate deteriorates a mechanical strength of asensor and has low reliability. Thus, it is difficult to achieve highsensitivity and high reliability of the sensor.

It could be helpful to provide an optical waveguide and an opticalconcentration measuring apparatus which realize high sensitivity andhigh reliability.

One of the disclosed aspects therefore provides an optical waveguideused in an optical concentration measuring apparatus for measuringconcentration of a target gas or a target liquid to be measured. Theoptical waveguide includes a substrate, a core layer that can transmitlight and extends along a longitudinal direction, a support thatconnects at least a portion of the substrate and a portion of the corelayer together and supports the core layer with respect to thesubstrate, and a protrusion that is arranged in a gap between thesubstrate and the core layer at a position different from a position ofthe support and protrudes toward the core layer from the substrate insuch a manner as to have a maximum height at a position deviated from acentral position of the core layer in a width direction.

One of the disclosed aspects therefore provides an optical concentrationmeasuring apparatus that includes the optical waveguide according to anyone of the embodiments of the present disclosure, a light source capableof irradiating light into the core layer, and a detector capable ofdetecting light transmitted through the core layer.

The present disclosure can provide an optical waveguide and an opticalconcentration measuring apparatus that realize high sensitivity and highreliability of a sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a diagram illustrating schematic configurations of an opticalwaveguide and an optical concentration measuring apparatus 1 accordingto a first embodiment of the present disclosure, for explaining sensingby the ATR method using the optical concentration measuring apparatus;

FIG. 2 is a plan view illustrating a part of a structure of the opticalwaveguide according to the first embodiment of the present disclosure,viewing a part of the optical waveguide from a light source side or aphotodetector side with respect to a principal surface of a substrate;

FIG. 3 is a cross-sectional end view of the optical waveguide taken fromline A-A in FIG. 1, for illustrating a schematic configuration of theoptical waveguide according to the first embodiment of the presentdisclosure;

FIG. 4 is a cross-sectional end view of the optical waveguide taken fromline B-B in FIG. 1, for illustrating a schematic configuration of theoptical waveguide according to the first embodiment of the presentdisclosure;

FIG. 5 is a plan view illustrating a portion of a structure of theoptical waveguide according to a variation of the first embodiment ofthe present disclosure, viewing a portion of the optical waveguide froma light source side or a photodetector side with respect to theprincipal surface of the substrate;

FIG. 6 is a plan view of a SOI substrate for illustrating a method formanufacturing the optical waveguide according to the first embodiment ofthe present disclosure;

FIG. 7 is a cross-sectional end view of the SOI substrate taken fromlines C-C and D-D in FIG. 6;

FIG. 8 is a plan view of an optical waveguide main portion forillustrating a method for manufacturing the optical waveguide accordingto the first embodiment of the present disclosure;

FIG. 9 is a cross-sectional end view of the optical waveguide mainportion taken from lines C-C and D-D in FIG. 8;

FIG. 10 is a plan view of the optical waveguide main portion partiallycovered with a mask pattern, for illustrating the method formanufacturing the optical waveguide according to the first embodiment ofthe present disclosure;

FIG. 11 is a cross-sectional end view of the optical waveguide mainportion taken from line C-C in FIG. 10;

FIG. 12 is a cross-sectional end view of the optical waveguide mainportion taken from line D-D in FIG. 10;

FIG. 13 is a plan view of the optical waveguide main portion in which aportion of a BOX layer is removed, for illustrating the method formanufacturing the optical waveguide according to the first embodiment ofthe present disclosure;

FIG. 14 is a cross-sectional end view of the optical waveguide mainportion taken from line C-C in FIG. 13;

FIG. 15 is a cross-sectional end view of the optical waveguide mainportion taken from line D-D in FIG. 13;

FIG. 16 is a cross-sectional end view of the optical waveguide mainportion taken from the same location as line C-C in the opticalwaveguide main portion illustrated in FIG. 13 according to a variationof the first embodiment of the present disclosure;

FIG. 17 is a diagram illustrating an evanescent wave of lighttransmitted through the optical waveguide;

FIG. 18 is a diagram illustrating an optical waveguide that includesintermittent supports having protrusions therebetween;

FIG. 19 is a plan view illustrating a result of observation analysis byan electron microscope, which includes the optical waveguide accordingto the first embodiment of the present disclosure, for illustratingdeviation between a top position of the protrusion in the widthdirection and a central position of the shadow of the core layer in thewidth direction;

FIG. 20 is a diagram illustrating a schematic configuration of anoptical waveguide and an optical concentration measuring apparatus 1according to a second embodiment of the present embodiment, forillustrating sensing employing ATR method using the opticalconcentration measuring apparatus;

FIG. 21 is a plan view illustrating a configuration of a portion of theoptical waveguide according to the second embodiment of the presentdisclosure, viewing a portion of the optical waveguide from a lightsource side or a photodetector side with respect to a principal surfaceof a substrate;

FIG. 22 is a cross-sectional end view of the optical waveguide takenfrom line E-E in FIG. 20, for illustrating a schematic configuration ofthe optical waveguide according to the second embodiment of the presentdisclosure;

FIG. 23 is a cross-sectional end view of the optical waveguide takenfrom line F-F in FIG. 20, for illustrating the schematic configurationof the optical waveguide according to the second embodiment of thepresent disclosure;

FIG. 24 is a cross-sectional end view of the optical waveguide takenfrom line G-G in FIG. 20, for illustrating the schematic configurationof the optical waveguide according to the second embodiment of thepresent disclosure;

FIG. 25 is a plan view of a SOI substrate, for illustrating a method formanufacturing the optical waveguide according to the second embodimentof the present disclosure;

FIG. 26 is a cross-sectional end view of a SOI substrate taken fromlines H-H, I-I, and J-J in FIG. 25;

FIG. 27 is a plan view of an optical waveguide main portion, forillustrating the method for manufacturing the optical waveguideaccording to the second embodiment of the present disclosure;

FIG. 28 is a cross-sectional end view of the optical waveguide mainportion taken from lines H-H, I-I, and J-J in FIG. 27;

FIG. 29 is a plan view of the optical waveguide main portion partiallycovered with mask patterns, for illustrating the method formanufacturing the optical waveguide according to the second embodimentof the present disclosure;

FIG. 30 is a cross-sectional end view of the optical waveguide mainportion taken from line I-I in FIG. 29;

FIG. 31 is a cross-sectional end view of the optical waveguide mainportion taken from line J-J in FIG. 29;

FIG. 32 is a plan view of the optical waveguide main portion in which aBOX layer is partially removed, for illustrating the method formanufacturing the optical waveguide according to the second embodimentof the present disclosure;

FIG. 33 is a cross-sectional end view of the optical waveguide mainportion taken from line H-H in FIG. 32;

FIG. 34 is a cross-sectional end view of the optical waveguide mainportion taken from line I-I line in FIG. 32;

FIG. 35 is a cross-sectional end view of the optical waveguide mainportion taken from line J-J line in FIG. 32;

FIG. 36 is a cross-sectional end view of an optical waveguide mainportion taken from the same position as line I-I in FIG. 32 according toa variation of the second embodiment of the present disclosure;

FIG. 37 is a diagram illustrating an optical waveguide that includesintermittent supports and protrusions arranged over the entire areabetween the intermittent supports; and

FIG. 38 is a diagram illustrating an experimental result for calculatingan area that does not include a support and a protrusion in alongitudinal direction of the core layer.

DETAILED DESCRIPTION

Hereinafter, embodiments of the present disclosure will be described.Note that the embodiments should not be construed as limiting theclaimed subject matter. Also, the solution to problem according to thepresent disclosure does not necessarily need all combinations offeatures described in the embodiments.

First Embodiment

Optical Waveguide

An optical waveguide according to a first embodiment of the presentdisclosure is an optical waveguide used in an optical concentrationmeasuring apparatus for measuring concentration of a target gas or atarget liquid to be measured. The optical waveguide includes asubstrate, a core layer that can transmit light and extends along alongitudinal direction, a support that connects at least a portion ofthe substrate and a portion of the core layer together and supports thecore layer with respect to the substrate, and a protrusion that isarranged at a position different from a position of the support in aspace between the substrate and the core layer and protrudes toward thecore layer from the substrate in such a manner as to have a maximumheight at a position deviated from a central position of the core layerin a width direction. Note that the longitudinal direction refers to adirection in which a three-dimensional structure having a shapeextending along at least one direction extends the most. Thelongitudinal direction includes a curved direction as well as a lineardirection. The position different from the position of the support inthe space between the substrate and the core layer refers to a positionin the space at which the support is not positioned in at least one ofthe longitudinal direction and the width direction. Also, the widthdirection refers to a direction perpendicular to the longitudinaldirection of the core layer and parallel to a principal surface of thesubstrate in the present embodiment. The principal surface of thesubstrate refers to a surface perpendicular to the thickness directionof the substrate, i.e., the largest surface in the six surfaces formingthe substrate. Further, the core layer may include a curved portion, andthe protrusion may be arranged in a space between the curved portion ofthe core layer and the substrate, i.e., immediately below the curvedportion of the core layer. The term “immediately below” refers to“straight under”, provided that a direction from the core layer to thesubstrate is defined as the vertical direction. The protrusion may bearranged immediately below the core layer close to an inner periphery oran outer periphery of the core layer in the curved portion of the corelayer.

Sensitivity of a sensor that employs the ATR method can be improved byincreasing an interaction amount between the evanescent wave and asubstance to be measured and reducing an amount of light absorbed by amaterial other than the substance to be measured. In order to reduce theamount of light absorbed by a material other than the substance to bemeasured, it is effective to arrange the supports for supporting thecore layer in a portion of the surface of the core layer opposing thesubstrate in such a manner as to expose the most portion of the corelayer. In a portion of the core layer that is not connected to thesupport and floats with respect to the substrate, sticking to thesubstrate may occur due to the meniscus force of water generated fromcondensation between the portion and the substrate. When such stickingoccurs, the structure having a floating portion in a part of thewaveguide cannot be maintained, and the sensitivity of the sensor issignificantly deteriorated. As such, arranging an object on the surfaceof the substrate can reduce the occurrence of sticking. Althoughsticking is often regarded as a problem in devices equipped withso-called MEMS (Microelectromechanical systems) that include movableparts, structures that do not include movable parts also need to preventsticking caused by the meniscus force as described above.

As an optical waveguide having this configuration, an optical waveguidethat includes a substrate, a core layer arranged on the substrate,support portions that connect the substrate and a portion of the corelayer together and intermittently support the core layer with respect tothe substrate, and a protrusion protruding toward the core layer fromthe substrate can be conceived. In the optical waveguide having such aconfiguration, the protrusions prevent the core layer from sticking tothe substrate, thus suppressing the occurrence of sticking. However,when the protrusion is arranged at the central position of the corelayer in the width direction for the purpose of reliable formation ofthe protrusion between the substrate and the core layer, there is a riskthat the evanescent wave extending with the maximum intensity in thevicinity of the central position of the core layer in the widthdirection is optically absorbed by the protrusion.

According to the optical waveguide of the present embodiment, theposition of the maximum height of the protrusion protruding toward thecore layer is deviated from the central position of the core layer inthe width direction. That is, a portion of the protrusion closest to thecore layer is located at a position deviated from the central position,through which the most part of light is transmitted, of the core layerin the width direction. According to the optical waveguide of thepresent embodiment, because the protrusion has the maximum height at aposition deviated from the central position of the core layer in thewidth direction as described above, optical absorption of the evanescentwave by the protrusion can be reduced, and occurrence of sticking of thecore layer can be suppressed. Thus, the optical concentration measuringapparatus that includes the optical waveguide according to the presentembodiment can reduce the possible failure due to sticking and improvethe measuring sensitivity and realize high sensitivity and highreliability. Also, the protrusion is arranged in a space between thecurved portion of the core layer and the substrate. Thus, the opticalwaveguide according to the present embodiment can suppress theoccurrence of sticking in the curved portion where sticking is morelikely to occur than in a linear portion. Further, in the curved portionof the core layer, the protrusion is arranged immediately below the corelayer at a position close to the inner periphery or the outer peripheryof the core layer. The protrusion is more preferably arranged at aposition close to the inner periphery of the core layer, because lighttransmitted through the curved portion of the core layer is slightlydeviated toward the outer periphery of the core layer, depending on theradius of curvature of the core layer. The optical waveguide in whichthe protrusion is arranged immediately below the core layer at aposition close to the inner periphery of the core layer in the curvedportion of the core layer increases the distance between the protrusionand the transmitted light, and thus can further reduce the opticalabsorption of the evanescent wave by the protrusion. However, an opticalwaveguide in which the protrusion is arranged immediately below the corelayer at a position close to the outer periphery of the core layer inthe curved portion of the core layer also increases the distance betweenthe protrusion and the transmitted light and thus has some effect. Notethat, in the optical waveguide according to the present embodiment, thecore layer does not need to include a movable portion.

Hereinafter, each constituent element of the optical waveguide will bedescribed using specific examples.

Core Layer

The core layer may be any layer that extends in the longitudinaldirection and can transmit light in the longitudinal direction. Inparticular, the core layer may be made of silicon (Si), gallium arsenide(GaAs), or germanium (Ge). The longitudinal direction refers to adirection in which a three-dimensional structure having a shapeextending along at least one direction extends the most and includes acurved direction as well as a linear direction. A vertical cross-sectionat a given position along the longitudinal direction of the core layeris not limited to have a circular shape and may have any shape,including a rectangle shape, in which the distance between the center ofthe cross-section and the outer surface varies when the cross-section isrotated about a central axis thereof. Accordingly, the core layer has anelongated plate-like shape in the present embodiment.

At least a portion of the core layer may be exposed to be able to comeinto direct contact with the target gas or the target liquid, or may becovered with a thin film thinner than ¼ of a wavelength, in vacuum, oflight transmitted through the core layer and be able to come intocontact with the target gas or the target liquid via the thin film. Thispromotes the interaction between the evanescent wave and the target gasor the target liquid and enables measurement of the concentration of thetarget gas or the target liquid.

Also, the core layer may include the curved portion. This enables theaspect ratio of the contour of the core layer to approach 1 when theentire core layer is viewed in plan view, and miniaturization of theoptical waveguide and the optical concentration measuring apparatus.Also, at least a portion of the core layer in the longitudinal directionincludes an area that does not include a support, which will be descriedlater, in the entire region between the core layer and the substrate ina cross-section perpendicular to the longitudinal direction. Thisconfiguration can promote the interaction between the evanescent waveextending from the core layer and the gas or the liquid surrounding theoptical waveguide. Note that “area that does not include the support”refers to a state in which the core layer forms a bridge between twosupports adjacent to each other in the longitudinal direction. Further,“area that does not include the support” refers to a state in which theentire region, opposing the substrate, of the core layer has a gap or amedium, having lower absorptivity with respect to the light transmittedthrough the core layer than the support between the substrate and theentire region, between two supports adjacent to each other in thelongitudinal direction.

The light transmitted through the core layer may be infrared radiationserving as an analog signal. Here, the infrared radiation serving as theanalog signal is not for determination on an energy change of lightbetween zero (low level) and 1 (high level) and represents a signal foran amount of the energy change of light. Thus, the optical waveguideaccording to each embodiment can be used for sensors and analysisequipment. In this case, the wavelength, in vacuum, of the infraredradiation may be 2 μm to 12 μm. This is a wavelength band absorbed bygasses (CO₂, CO, NO, N₂O, SO₂, CH₄, H₂O, C₂H₆O, and the like) that aretypically floating in the environment. Accordingly, the opticalwaveguide according to each embodiment can be used as a gas sensor.

Substrate

The substrate may be any substrate on which the support, the protrusion,and the core layer can be formed. In particular, the substrate may be asilicon substrate or a GaAs substrate. The principal surface of thesubstrate refers to the surface of the substrate extending in ahorizontal direction (a direction perpendicular to the film thicknessdirection). The surface of the substrate does not need to be exposed andmay be partially covered with a thin film made of the same material asthe support and the protrusion. When the surface of the substrate is notexposed, the height of the protrusion described later refers to theheight of the surface of the protrusion from the lowest position of thethin film surface in the area immediately below the core layer, ratherthan the height of the surface of the protrusion from the surface of thesubstrate.

Support

The support connects at least a portion of the substrate and a portionof the core layer together. The support supports the core layer withrespect to the substrate.

The support may be any support capable of connecting the substrate andthe core layer together and is preferably made of a material that has asmaller refractive index than the core layer with respect to light ofany wavelength or light transmitted through the core layer. The materialof the support may be, for example, silicon oxide film (SiO2), silicon(Si), gallium arsenide (GaAs), or the like. The index of refraction isthe index of refraction with respect to light of any wavelength or lightof a particular wavelength. The light of a particular wavelength is, inparticular, the light transmitted through the core layer of the opticalconcentration measuring apparatus. In this way, the support can reflectthe entire light transmitted through the core layer at the connectingportion connected to the core layer.

The connecting portion between the support and the core layer may bepositioned at any position on the surface, opposing the substrate, ofthe core layer including, for example, a central portion in the widthdirection. Also, the connecting portion between the support and the corelayer may be at any position on the end surface of the core layer in thewidth direction. Also, the connecting portion between the support andthe core layer may be intermittently arranged along the longitudinaldirection of the core layer. This configuration enlarges the outersurface that does not contact the support in a portion of the core layerin the longitudinal direction, and thus can enlarge an interaction areabetween the evanescent wave and the target gas or the target liquid.Further, the connecting portion between the support and the core layermay have a shape extending in the longitudinal direction of the corelayer as it is positioned closer to the central position of the corelayer away from the edge portion of the core layer in the widthdirection. By virtue of this shape, the surrounding conditions of thecore layer gradually change along the longitudinal direction of the corelayer from an area that does not include the support to an area thatincludes the support (or vice versa). Thus, a sudden change of thesurrounding conditions for the light transmitted through the core layercan be avoided, and a scattering loss of the light transmitted throughthe core layer can be reduced.

A method for forming the support is, for example, etching a BOX (BuriedOxide) layer in the SOI (Silicon On Insulator) substrate, whereby astructure in which the core layer (Si layer) is supported by the BOXlayer with respect to the substrate (Si layer) can be formed.

Protrusion

The protrusion is arranged at a position different from a position ofthe support in a space between the substrate and the core layer. Theprotrusion has a maximum height at a position deviated from the centralposition of the core layer in the width direction. Note that theprotrusion does not need to have a maximum height at a position deviatedfrom the central position of the core layer in the width direction overthe entire region of the core layer in the longitudinal direction. Forexample, the protrusion may have a maximum height at a position deviatedfrom the central position of the core layer in the width direction atleast in a portion of the core layer in the longitudinal direction, andhave a maximum height at the central position of the core layer in thewidth direction in another portion. The protrusion protrudes toward thecore layer from the substrate. “The position different from the positionof the support between the substrate and the core layer” refers to aposition in a space between the substrate and the core layer that doesnot include the support at least in one of the longitudinal directionand the width direction.

The protrusion may be formed from the same material as the support. Thisfacilitates the formation of the protrusion arranged in an areaoverlapping with the support in the thickness direction of the corelayer by employing the lithography technology and the etchingtechnology. Also, the protrusion may have a mountain-like shape. Thus,in a configuration in which the core layer has, for example, arectangular shape in a plane perpendicular to the longitudinaldirection, the bottom surface of the core layer and the surface of theprotrusion having the mountain-like shape are not in parallel with eachother, and sticking between the core layer and the protrusion isunlikely to occur. When the protrusion does not have a mountain-likeshape, a similar effect can be obtained by virtue of that the bottomsurface of the core layer and the surface of the protrusion opposing thecore layer are not in parallel with each other. Note that themountain-like shape of the protrusion is a shape of the protrusionviewed from any direction perpendicular to the thickness direction ofthe core layer, e.g., from the longitudinal direction of the core layer.Although the protrusion may have an asymmetrical shape in the widthdirection of the core layer, the protrusion preferably has a symmetricalshape. The symmetrical shape in the width direction of the core layermeans a line-symmetrical shape with a straight line perpendicular to thewidth direction serving as the axis of symmetry. Thus, in aconfiguration in which the core layer is symmetrical in the widthdirection and the protrusion is also symmetrical in the width direction,when light transmits to the core layer in an area that includes theprotrusion immediately below the core layer from the care layer in anarea that does not include the protrusion immediately below the corelayer, mode conversion of light is suppressed, and transmission loss dueto the mode conversion is reduced. The protrusion may have a maximumheight at a position deviated from the central position of the corelayer in the width direction by 3% or more of the width of the corelayer, preferably by 10% or more, more preferably by 20% or more, andfurther preferably by 30% or more. Also, the protrusion may have amaximum height at a position deviated from the central position of thecore layer in the width direction by 100 nm or more, preferably by 300nm or more, more preferably by 600 nm or more, and further preferably by900 nm or more. Thus, a ratio of the extended evanescent wave thatreaches the protrusion can be reduced. The more the position of themaximum height of the protrusion is deviated from the central positionof the core layer in the width direction, the more the opticalabsorption of the evanescent wave by the protrusion can be reduced. Themaximum height of the protrusion may be 1/20 or more of the distancebetween the core layer and the substrate. This configuration cansuppress the occurrence of sticking of the core layer. The maximumheight of the protrusion may be 100 nm or more. This configuration canfurther suppress the occurrence of sticking of the core layer. Further,the protrusions may be intermittently arranged along the longitudinaldirection of the core layer. This configuration reduces a total lengthof the protrusion in the longitudinal direction, and thus can furthersuppress the optical absorption of the evanescent wave by theprotrusion.

Optical Concentration Measuring Apparatus

An optical concentration measuring apparatus according to the presentembodiment includes the optical waveguide according to the presentembodiment, a light source capable of irradiating light into the corelayer, and a detector capable of detecting light transmitted through thecore layer.

Each constituent element of the optical concentration measuringapparatus will be described below with specific examples.

Light Source

The light source may be any light source capable of irradiating lightinto the core layer. When infrared radiation is used for the measurementof a gas, an incandescent bulb, a ceramic heater, a MEMS(Microelectromechanical systems) heater, an infrared LED (Light-EmittingDiode), or the like can be used as the light source. The light sourcemay be arranged in any manner that can be optically connected to theoptical waveguide. For example, the light source may be arrangedadjacent to the optical waveguide in the same constituent element, ormay be arranged at a certain distance from the optical waveguide as adifferent constituent element. When ultraviolet rays are used for themeasurement of a gas, a mercury lamp, an ultraviolet LED, or the likecan be used as the light source. When x-rays are used for themeasurement of a gas, an electron beam, an electron laser, or the likecan be used as the light source.

The light transmitted through the core layer of the optical waveguideprovided in the optical concentration measuring apparatus may beinfrared radiation serving as an analog signal. Here, the infraredradiation serving as the analog signal is not for determination on anenergy change of light between zero (low level) and 1 (high level), andrepresents a signal for an amount of the energy change of light. Thus,the optical concentration measuring apparatus can be used for sensorsand analysis equipment. In this case, the wavelength, in vacuum, of theinfrared radiation may be 2 μm to 12 μm. This is the wavelength bandabsorbed by gasses (CO₂, CO, NO, N₂O, SO₂, CH₄, H₂O, C₂H₆O, and thelike) that are typically floating in the environment. Accordingly, theoptical concentration measuring apparatus according to the presentembodiment can be used as a gas sensor.

Detector

The detector may be any detector capable of receiving light transmittedthrough the core layer of the optical waveguide. When infrared radiationis used for the measurement of a gas, a thermal infrared sensor such asa pyroelectric sensor, a thermopile, or a bolometer, or a quantuminfrared sensor such as a diode or a phototransistor may be used as thedetector. When ultraviolet rays are used for the measurement of a gas, aquantum ultraviolet sensor such as a diode or a phototransistor may beused as the detector. When x-rays are used for the measurement of a gas,various types of a semiconductor sensor may be used as the detector.

The optical waveguide and the optical concentration measuring apparatusaccording to the present embodiment will be described with reference toFIG. 1 to FIG. 15.

FIG. 1 is a diagram illustrating a schematic configuration of an opticalconcentration measuring apparatus 1 according to the present embodimentand is also a conceptual drawing of the ATR method using an opticalwaveguide 10 according to the present embodiment.

As illustrated in FIG. 1, the optical concentration measuring apparatus1 is installed and used in an external space 2 that includes a gas whoseconcentration or the like is to be detected. The optical concentrationmeasuring apparatus 1 includes the optical waveguide 10 according to thepresent embodiment, a light source 20 capable of irradiating light(infrared radiation IR in the present embodiment) into a core layer 11provided in the optical waveguide 10, and a photodetector (an example ofa detector) 40 capable of receiving the infrared radiation IRtransmitted through the core layer 11.

The optical waveguide 10 includes a substrate 15, the core layer 11 thatcan transmit the infrared radiation IR (an example light), supports 17each of which intermittently connects at least a portion of thesubstrate 15 and a portion of the core layer 11 together and thussupports the core layer 11 with respect to the substrate 15, andprotrusions 18 that protrude toward the core layer 11 from the substrate15. The core layer 11 and the substrate 15 are made of silicon (Si), andthe supports 17 and the protrusions 18 are made of silicon dioxide(SiO₂).

The substrate 15 has, for example, a plate-like shape. The core layer 11is, for example, a rectangular parallelepiped in part. The core layer 11includes a curved portion in which the rectangular parallelepiped iscurved about an axis in a direction perpendicular to a principle surfaceof the substrate 15, as illustrated in FIG. 2.

As illustrated in FIG. 1, the optical waveguide 10 includes a gratingcoupler 118 formed at one end of the core layer 11 in the longitudinaldirection and a grating coupler 119 formed at the other end of the corelayer 11 in the longitudinal direction. The grating coupler 118 isarranged in the irradiating direction of the light source 20. In thepresent embodiment, the optical waveguide 10 is arranged such that thestacking direction therein is parallel to the vertical direction, andthe principle surface of the substrate 15 is directed in the verticallydownward direction. The irradiating direction of the light source 20 isvertically downward from the light source 20 in a state in which theoptical waveguide 10 is installed in the above manner. The gratingcoupler 118 couples the infrared radiation IR irradiated by the lightsource 20 with the infrared radiation IR transmitted through the corelayer 11. The grating coupler 119 is arranged in a direction facing thephotodetector 40. Note that “direction facing the photodetector 40” is adirection vertically downward from the photodetector 40 in a state inwhich the optical waveguide 10 is installed in the above manner. Thegrating coupler 119 is configured to extract the infrared radiation IRtransmitted through the core layer 11 and pass the extracted infraredradiation IR to the photodetector 40.

FIG. 3 is a cross-sectional end view of the optical waveguide 10 takenfrom line A-A in FIG. 1, and FIG. 4 is a cross-sectional end view of theoptical waveguide 10 taken from line B-B in FIG. 1.

As illustrated in FIG. 1, FIG. 3, and FIG. 4, the optical waveguide 10includes gaps 13 between the core layer 11 and the substrate 15 withouthaving a predetermined layer such as a clad layer, except for the areasthat include the supports 17.

As illustrated in FIG. 3, a connecting portion 171 of the support 17connected to the core layer 11 overlaps with the central position cp ofthe core layer 11 in the width direction. As illustrated in FIG. 1,also, the connecting portions 171 of the supports 17 are intermittentlyarranged along the longitudinal direction.

As illustrated in FIG. 1 and FIG. 2, in the longitudinal direction ofthe core layer 11, the protrusions 18 are arranged at positionsdifferent from positions of the supports 17 in the gap between thesubstrate 15 and the core layer 11. As illustrated in FIG. 3 and FIG. 4,also, in the width direction of the core layer 11, the protrusions 18are arranged at positions different from positions of the supports 17 inthe gap. Each of the protrusions 18 has a maximum height at a positiondeviated from the central position cp of the core layer 11 in the widthdirection. Each of the protrusions 18 has a mountain-like shape asviewed from the longitudinal direction of the core layer 11. Each of theprotrusions 18 has a symmetrical shape in the width direction of thecore layer 11. Each of the protrusions 18 has a maximum height at aposition deviated from the central position cp of the core layer in thewidth direction by 3% or more of the width of the core layer 11. Themaximum height of the protrusions 18 is 1/20 or more of a gap gp betweenthe core layer 11 and the substrate 15, which is 100 nm or more, inparticular. As illustrated in FIG. 1 and FIG. 2, the protrusion 18 isalso arranged immediately below the curved portion of the core layer 11along the longitudinal direction. In the curved portion of the corelayer 11, the protrusion 18 is arranged immediately below the core layer11 at a position deviated toward the inner periphery or the outerperiphery of the core layer 11 (toward the inner periphery in FIG. 2).In the curved portion of the core layer 11, the protrusion 18 extendsalong the curved portion of the core layer 11 as viewed from thethickness direction of the core layer 11.

Note that the protrusion 18 arranged in the curved portion of the corelayer 11 does not need to extend along the curve of the core layer 11.As illustrated in FIG. 5, for example, the protrusion 18 may be curvedhaving a radius of curvature different from that of the curved portionand arranged such that a portion of the protrusion 18 is close to theinner periphery or the outer periphery of the core layer 11.

The shape of the protrusions 18 and the positions of the protrusions 18with respect to the width direction of the core layer 11 can be observedin the following manner. To observe a cross-sectional shape of aprotrusion 18, a SEM (scanning electron microscope) is used to observe aplane perpendicular to the longitudinal direction. To observe a relativeposition between the protrusion 18 and the core layer 11 in the widthdirection, a damaged layer DM is formed by emitting an FIB (Focused IonBeam) from a side of the core layer 11 of the optical waveguide 10 suchthat a shadow WS of the core layer 11 is transferred onto the substrate.Then, the core layer 11 is peeled off, and the shadow WS, transferred tothe peeled surface, of the core layer 11 and the protrusion 18 areobserved using the SEM. In particular, the FIB is emitted in thedirection perpendicular to the principle surface of the substrate 15 ata portion where the core layer 11 is floating. Thus, the shadow WS ofthe core layer 11 is transferred onto the substrate 15 by the damagedlayer DM generated by the FIB. The transferred shadow WS of the corelayer 11 indicates the position of the core layer 11 in a plane parallelto the principle surface of the substrate 15. Thus, by comparing thetransferred shadow WS of the core layer 11 and the position of theprotrusion 18, the relative position between the protrusion 18 and thecore layer 11 can be determined. FIG. 19 illustrates the shadow WS ofthe core layer 11 and the protrusion 18 actually observed by employingthe above method. When the principle surface of the substrate 15 isobserved using the SEM, there is a clear difference in the contrastsbetween an area where the damaged layer DM generated by the FIB isformed and an area of the shadow WS, where the damaged layer DM was notgenerated by the FIB, of the core layer 11. It was observed that therewas a 300 nm deviation in the width direction between the lineindicating the peak of the protrusion 18 having the mountain-like shapeand the central position cp of the shadow WS, transferred onto thesubstrate 15, of the core layer 11 in the width direction. That is, itcan be seen that the protrusion 18 is formed protruding toward the corelayer 11 from the substrate 15 and has a maximum height at a positiondeviated from the central position cp of the core layer 11 in the widthdirection.

Here, an effect of the optical waveguide 10 according to the presentembodiment will be described by comparing with an optical waveguide 10′that includes a protrusion 18′ arranged at the central position cp inthe width direction as illustrated in FIG. 18.

A sensor that employs the ATR method is often set to transmit light in asingle mode within the core layer. In the example of the opticalconcentration measuring apparatus 1 according to the present embodiment,light (infrared radiation) is transmitted in the single mode within thecore layer 11 provided in the optical waveguide 10. Note that, whenlight is transmitted in a multi-mode, some light components aretransmitted through the center of the core layer, whereby the effect ofthe present disclosure can be obtained. As illustrated in FIG. 3 andFIG. 4, when the infrared radiation IR is transmitted through the corelayer 11 in the single mode, the optical axis OA of the infraredradiation IR is positioned at an approximate center of the core layer 11in a cross-section orthogonal to the longitudinal direction, which isthe transmission direction of the infrared radiation IR. At this time,an amount of an evanescent wave EW extending around the core layer 11increases near the outer surface of the core layer 11 close to theoptical axis OA. Thus, the amount of the evanescent wave EW reachesmaximum in the vicinity of the central position cp in the widthdirection in the space between the core layer 11 and the substrate 15.The distribution of the evanescent wave EW of the infrared radiation IRtransmitted through the core layer 11′ of the optical waveguide 10′ thatincludes the protrusion 18′ at the central position cp in the widthdirection as illustrated in FIG. 18 is similar to the distributionobtained using the optical waveguide 10 of the present embodiment.

In a sensor employing the ATR method, the sensitivity of the sensor isimproved by expanding a region of interaction between the evanescentwave extending from the core layer and the substance to be measured(that is, by increasing an exposed portion of the core layer) andsuppressing optical absorption by materials other than the substance tobe measured (that is, absorption of light by the support or the like).Arranging the protrusion 18′ formed for the purpose of suppressingoccurrence of sticking at the central position cp of the core layer 11′in the width direction as illustrated in FIG. 18 can be considered as asimplest configuration.

In the optical waveguide 10′, however, because the protrusion 18′ isarranged at the central position cp of the core layer 11′ in the widthdirection between the core layer 11′ and the substrate 15′ in a planeorthogonal to the longitudinal direction, i.e., in the transmittingdirection of the light L (i.e., in the cross-section illustrated in FIG.18), the evanescent wave EW is absorbed most significantly by thematerial forming the protrusion 18′. As a result, the sensitivity of thesensor using the optical waveguide 10′ is deteriorated.

As illustrated in FIG. 2 and FIG. 4, the optical waveguide 10 accordingto the present embodiment has a structure similar to the opticalwaveguide 10′, in which the protrusions 18 are provided in the spacebetween the core layer 11 and the substrate 15 to suppress theoccurrence of sticking. In a cross-section perpendicular to thelongitudinal direction, the core layer 11 has a symmetrical structurewith respect to the center thereof. When the infrared radiation IRtransmitted through the core layer 11 is in the single mode, the opticalaxis OA of the infrared radiation IR transmitted through the core layer11 overlaps with the center of the core layer 11. As such, theprotrusion 18 is positioned deviated from the central position cp of thecore layer 11 in the width direction toward one of the peripheries asillustrated in FIG. 2. Thus, the protrusion 18 can be positioneddeviated from the area where the evanescent wave EW is mostconcentrated. That is, the position of the maximum height of theprotrusion 18 is not positioned at the central position cp of the corelayer 11 in the width direction in the cross-section perpendicular tothe longitudinal direction. Further, the core layer 11 includes thecurved portion for the purpose of miniaturization of the opticalwaveguide 10 in its entirety, and the protrusion 18 is arrangedimmediately below the core layer 11 in the curved portion. Thus, theoptical waveguide 10 can enhance durability against sticking in thecurved portion, which otherwise has lower durability against stickingthan the linear portion, of the core layer. Further, the protrusion 18is disposed immediately below the core layer 11 at a position close tothe inner periphery or the outer periphery of the core layer 11 in thecurved portion of the core layer 11 (close to the inner periphery inFIG. 2). In the curved portion of the core layer 11, the lighttransmitted therethrough is located slightly close to the outerperiphery of the core layer 11, depending on the radius of curvature ofthe core layer 11. Thus, it is preferable to arrange the protrusion 18at a position close to the inner periphery of the core layer 11. In theoptical waveguide 10 having a configuration in which the protrusion 18is arranged at a position close to the inner periphery immediately belowthe core layer 11 in the curved portion, the distance between theprotrusion 18 and the infrared radiation IR transmitted through the corelayer 11 is increased. Thus, the optical absorption of the evanescentwave EW by the protrusion 18 can be further reduced. However, in aconfiguration in which the protrusion 18 is disposed close to the outerperiphery of the core layer 11, the distance between the protrusion 18and the infrared radiation IR transmitted through the core layer 11 isincreased in a similar manner, whereby the same effect can be obtainedto a certain extent.

Next, a method for manufacturing the optical waveguide 10 according tothe present embodiment will be described with reference to FIG. 6 toFIG. 16 using FIG. 1 to FIG. 4. FIG. 6 illustrates a manufacturingprocess plan view of the optical waveguide 10. FIG. 7 illustrates across-sectional manufacturing process end view of the optical waveguide10 taken from lines C-C and D-D in FIG. 6. FIG. 8 illustrates amanufacturing process plan view of the optical waveguide 10. FIG. 9illustrates a cross-sectional manufacturing process end view of theoptical waveguide 10 taken from lines C-C and D-D in FIG. 8. FIG. 10illustrates a manufacturing process plan view of the optical waveguide10. FIG. 11 illustrates a cross-sectional manufacturing process end viewof the optical waveguide 10 taken from line C-C in FIG. 10. FIG. 12illustrates a manufacturing process plan view of the optical waveguide10 taken from line D-D in FIG. 10. FIG. 13 illustrates a manufacturingprocess plan view of the optical waveguide 10. FIG. 14 illustrates across-sectional manufacturing process end view of the optical waveguide10 taken from line C-C in FIG. 13. FIG. 15 illustrates a cross-sectionalmanufacturing process end view of the optical waveguide 10 taken fromline D-D in FIG. 13. FIG. 16 illustrates a cross-sectional manufacturingend view taken from the same position as the line C-C in the opticalwaveguide main portion 10 a illustrated in FIG. 13.

First, a SiO₂ film is formed on one or both of a support substrate 15 athat is made of silicon and eventually formed as the substrate 15, andan active substrate 11 a that is made of silicon and eventually formedas the core layer 11. Next, the support substrate 15 a and the activesubstrate 11 a are stuck together with the SiO₂ film therebetween andsubjected to heat treatment to be joined together. Then, the filmthickness of the active substrate 11 a is adjusted to a predeterminedthickness by grinding, polishing, or the like. Thus, an SOI substrate100 having a “silicon-insulating layer-silicon” structure that includesthe support substrate 15 a, a BOX layer 17 a formed on the supportsubstrate 15 a, and the active substrate 11 a formed on the BOX layer 17a is formed as illustrated in FIG. 6 and FIG. 7.

Next, lithography and etching are performed on the SOI substrate 100 toetch the active substrate 11 a and form the core layer 11 that has arectangular cross-section and extends in the longitudinal direction.Thus, the optical waveguide main portion 10 a that includes a supportsubstrate 15 a having a plate-like shape, the BOX layer 17 a having aplate-like shape and formed on the support substrate 15 a, and the corelayer 11 formed as a rectangular prism on a portion of the BOX layer 17a is formed as illustrated in FIG. 8 and FIG. 9.

Next, a mask pattern M1 and a mask pattern M2 that cover a portion ofthe core layer 11 and a portion of the BOX layer 17 a, respectively, areformed as illustrated in FIG. 10 to FIG. 12. As illustrated in FIG. 10and FIG. 11, the mask pattern M1 is arranged in a manner deviated fromthe central position of the core layer 11 in the width direction towardone of the peripheries. Also, in the curved portion of the core layer11, the mask pattern M1 is disposed close to the inner periphery or theouter periphery (the inner periphery in FIG. 10 and FIG. 11). The maskpattern M2 is arranged at the central position of the core layer 11 inthe width direction and wider than the mask pattern M1 as illustrated inFIG. 10 and FIG. 12. The mask pattern M1 and the mask pattern M2 may beformed from photoresist or may be hard masks such as silicon nitridefilms. The mask pattern M1 and the mask pattern M2 may be an integralmask layer (i.e., one mask layer) or individual mask layers.

Next, a portion of the BOX layer 17 a of the optical waveguide mainportion 10 a is removed by performing wet etching or the like using themask pattern M1 and the mask pattern M2 as masks. Thus, the protrusion18 is formed in a manner deviated from the central position of the corelayer 11 in the width direction toward the inner periphery or the outerperiphery of the curved portion (toward the inner periphery in FIG. 13and FIG. 14) as illustrated in FIG. 13 and FIG. 14. Also, the support 17is formed at the central position cp of the core layer 11 in the widthdirection as illustrated in FIG. 13 and FIG. 15. Although a portion ofthe BOX layer 17 a other than the portion forming the support 17 and theprotrusion 18 is removed in FIG. 13 to FIG. 15, this portion of the BOXlayer 17 a does not need to be completely removed. As illustrated in avariation of the present embodiment referring to in FIG. 16, a thin film19 covering the support substrate 15 a may be formed from the remainderof the BOX layer 17 a. At this time, the maximum height of theprotrusion 18 corresponds to the height of the peak of the mountain-likeshape of the protrusion 18 from a lowest position of the surface of thethin film 19 in the region immediately below the core layer 11.

Next, the mask pattern M1 and the mask pattern M2 are etched. Althoughthe formation of the grating couplers is omitted in the presentembodiment, when the grating couplers 118 and 119 as illustrated in FIG.1 are formed, the grating couplers 118 and 119 may be formedsimultaneously with, before, or after the formation of the core layer 11illustrated in FIG. 8, followed by the formation of the mask patterns M1and M2 illustrated in FIG. 10. The configuration illustrated in FIG. 1is obtained by forming the grating coupler 118 in a slit-like shape atone end portion of the core layer 11 in the longitudinal direction andforming the grating coupler 119 having a slit-like shape at the otherend portion of the core layer 11 in the longitudinal direction.

Then, the support substrate 15 a is cut in a predetermined region toseparate the optical waveguide main portion 10 a. Thus, the opticalwaveguide 10 in which the protrusion 18 is deviated from the centralposition cp of the core layer 11 in the width direction is produced (seeFIG. 1 to FIG. 4).

As illustrated in FIG. 1, further, the optical concentration measuringapparatus 1 is produced by installing the light source 20 in such amanner as to be able to irradiate the infrared radiation IR into thegrating coupler 118 of the optical waveguide 10 and by arranging thephotodetector 40 in such a manner as to be able to receive the infraredradiation IR exit from the grating coupler 119 of the optical waveguide10.

As described above, the optical waveguide 10 has a configuration inwhich the protrusion 18 is deviated from the central position cp of thecore layer 11 in the width direction toward one of the peripheries.Thus, the optical waveguide 10 can suppress the occurrence of stickingof the core layer 11 and prevent deterioration of detectingcharacteristics for detecting a substance to be measured MO caused bythe protrusion 18.

According to the present embodiment, as described above, the opticalwaveguide 10 and the optical concentration measuring apparatus 1 thatinclude the protrusion 18 which suppresses the occurrence of sticking ofthe core layer 11 and prevent deterioration of the sensitivity of thesensor can be provided.

Further, the optical waveguide 10 according to the present embodimentcan reduce the amount of the evanescent wave EW absorbed by theprotrusion 18. Thus, the optical waveguide 10 according to the presentembodiment can detect the substance to be measured MO with highsensitivity in a stable manner in various specification modes.

Second Embodiment

Optical Waveguide

An optical waveguide according to a second embodiment of the presentdisclosure is an optical waveguide used in an optical concentrationmeasuring apparatus for measuring concentration of a target gas or atarget liquid to be measured. The optical waveguide includes asubstrate, a core layer that extends along a longitudinal direction andis capable of transmitting light, a support that connects at least aportion of the substrate and a portion of the core layer together andsupports the core layer with respect to the substrate, and protrusionsthat are intermittently arranged in the longitudinal direction in thespace between the substrate and core layer and protruding toward thecore layer from the substrate. Note that the longitudinal directionrefers to a direction in which a three-dimensional structure having ashape extending along at least one direction extends the most andincludes a curved direction as well as a linear direction. The corelayer may include a curved portion, and a protrusion may be arranged ina space between the curved portion of the core layer and the substrate,i.e., immediately below the curved portion of the core layer. The term“immediately below” refers to “straight under”, provided that adirection from the core layer to the substrate is defined as thevertical direction. The position of the maximum height of the protrusionin the width direction of the core layer may be deviated from thecentral position of the core layer. In the present embodiment, the term“width direction” refers to a direction that is perpendicular to thelongitudinal direction of the core layer and parallel to the principlesurface of the substrate. The principal surface of the substrate refersto a surface perpendicular to the thickness direction of the substrate,i.e., the largest surface in the six surfaces forming the substrate.

In the sensor employing the ATR method, the sensor sensitivity can beimproved by increasing an interaction amount between the evanescent waveand the substance to be measured and reducing an amount of lightabsorbed by a material other than the substance to be measured. In orderto reduce the amount of light absorbed by a material other than thesubstance to be measured, it is effective to arrange the supports forsupporting the core layer in a portion of the surface of the core layeropposing the substrate in such a manner as to expose the most portion ofthe core layer. In a portion of the core layer that is not connected tothe support and floats with respect to the substrate, sticking to thesubstrate may occur due to the meniscus force of water generated fromcondensation between the portion and the substrate. When such stickingoccurs, the structure having a floating portion in a part of thewaveguide cannot be maintained, and the sensitivity of the sensor issignificantly deteriorated. As such, arranging an object on the surfaceof the substrate can reduce the occurrence of sticking. Althoughsticking is often regarded as a problem in devices equipped withso-called MEMS (Microelectromechanical systems) having movable parts,structures that do not include movable parts also need to preventsticking caused by the meniscus force as described above.

As an optical waveguide having this configuration, an optical waveguidethat includes a substrate, a core layer arranged on the substrate,support portions that connect the substrate and a portion of the corelayer together and intermittently support the core layer with respect tothe substrate, and a protrusion protruding toward the core layer fromthe substrate can be conceived. In the optical waveguide having such aconfiguration, the protrusions prevent the core layer from sticking tothe substrate, thus suppressing the occurrence of the sticking. However,because the protrusion is reliably formed between the substrate and thecore layer, there is a risk that the evanescent wave extending from thecore layer is optically absorbed by the protrusion.

According to the optical waveguide of the present embodiment, theprotrusions are intermittently arranged in the longitudinal direction ofthe core layer. That is, there are areas between the core layer and thesubstrate that do not include the protrusion in the longitudinaldirection. Thus, the optical waveguide of the present embodiment cansuppress the occurrence of sticking of the core layer in the areas thatinclude the protrusions and also reduce the amount of the evanescentwave optically absorbed by the protrusions in the areas that do notinclude the protrusions. Thus, the optical concentration measuringapparatus that includes the optical waveguide according to the presentembodiment can improve the measurement sensitivity while reducing thelikelihood of failure of the optical waveguide caused by sticking andrealize high sensitivity and high reliability. Also, the protrusion isarranged in a space between the curved portion of the core layer and thesubstrate. Thus, the optical waveguide according to the presentembodiment can suppress the occurrence of sticking in the cured portionwhere sticking is more likely to occur than in a linear portion.Further, the position of the maximum height of the protrusion in thewidth direction is deviated from the central position of the core layerin the width direction. That is, a portion of the protrusion positionedclosest to the core layer is deviated from the center, where the mostpart of the light is transmitted, of the core layer in the widthdirection. As described above, in the optical waveguide according to thepresent embodiment as described above, the protrusion has the maximumheight at a position deviated from the central position of the corelayer in the width direction. Thus, the amount of the evanescent waveoptically absorbed by the protrusion can be further reduced, and themeasuring sensitivity can be further improved. Note that, in the opticalwaveguide according to the present embodiment, the core layer does notneed to include a movable portion.

Hereinafter, each constituent element of the optical waveguide will bedescribed using specific examples.

Core Layer

The core layer may be any layer that extends along the longitudinaldirection and is capable of transmitting light in the longitudinaldirection. In particular, the core layer may be made of silicon (Si),gallium arsenide (GaAs), or germanium (Ge). The longitudinal directionrefers to a longest extending direction of a three-dimensional structurehaving a shape extending along at least one direction and includes acurved direction as well as a linear direction. The longitudinaldirection may refer to a linear direction and a curved direction. Avertical cross-section at a given position along the longitudinaldirection of the core layer is not limited to have a circular shape andmay have any shape, including a rectangle shape, in which the distancebetween the center of the cross-section and the outer surface varieswhen the cross-section is rotated about a central axis thereof.Accordingly, the core layer has an elongated plate-like shape in thepresent embodiment.

At least a portion of the core layer may be exposed to be able to comeinto direct contact with a target gas or a target liquid, or may becovered with a thin film thinner than ¼ of a wavelength, in vacuum, oflight transmitted through the core layer and be able to come intocontact with the target gas or the target liquid via the thin film. Thispromotes the interaction between the evanescent wave and the target gasor the target liquid and enables measurement of the concentration of thetarget gas or the target liquid.

Also, the core layer may include the curved portion. This enables theaspect ratio of the contour of the core layer to approach 1 when theentire core layer is viewed in plan view, and miniaturization of theoptical waveguide and the optical concentration measuring apparatus.Also, at least a portion of the core layer in the longitudinal directionincludes a region that does not include a support, which will bedescried later, in the entire region between the core layer and thesubstrate in a cross-section perpendicular to the longitudinaldirection. This configuration can promote the interaction between theevanescent wave extending from the core layer and the gas or the liquidsurrounding the optical waveguide. Note that “area that does not includethe support” refers to a state in which the core layer forms a bridgebetween two supports adjacent to each other in the longitudinaldirection. Further, “area that does not include the support” refers to astate in which the entire region, opposing the substrate, of the corelayer has a gap or a medium, having lower absorptivity with respect tothe light transmitted through the core layer than the support betweenthe substrate and the entire region, between two supports adjacent toeach other in the longitudinal direction.

The light transmitted through the core layer may be infrared radiationserving as an analog signal. Here, the infrared radiation serving as theanalog signal is not for determination on an energy change of lightbetween zero (low level) and 1 (high level), and represents a signal foran amount of the energy change of light. Thus, the optical waveguideaccording to each embodiment can be used for sensors and analysisequipment. In this case, the wavelength, in vacuum, of the infraredradiation may be 2 μm to 12 μm. This is the wavelength band absorbed bygasses (CO₂, CO, NO, N₂O, SO₂, CH₄, H₂O, C₂H₆O, and the like) that aretypically floating in the environment. Accordingly, the opticalwaveguide according to each embodiment can be used as a gas sensor.

Substrate

The substrate may be any substrate on which the support, the protrusion,and the core layer can be formed. In particular, the substrate may be asilicon substrate or a GaAs substrate. The principal surface of thesubstrate refers to the surface of the substrate extending in ahorizontal direction (a direction perpendicular to the film thicknessdirection). The surface of the substrate does not need to be exposed andmay be partially covered with a thin film made of the same material asthe support and the protrusion. When the surface of the substrate is notexposed, the height of the protrusion described later refers to theheight of the surface of the protrusion from the lowest position of thethin film surface in the region immediately below the core layer, ratherthan the height of the surface of the protrusion from the surface of thesubstrate.

Support

The support connects at least a portion of the substrate and a portionof the core layer together. The support supports the core layer withrespect to the substrate.

The support may be any support capable of connecting the substrate andthe core layer together and is preferably made of a material that has asmaller refractive index than the core layer with respect to light ofany wavelength or light transmitted through the core layer. The materialof the support may be, for example, silicon oxide film (SiO2), silicon(Si), gallium arsenide (GaAs), or the like. The index of refraction isthe index of refraction with respect to light of any wavelength or lightof a particular wavelength. The light of a particular wavelength is, inparticular, the light transmitted through the core layer of the opticalconcentration measuring apparatus. In this way, the support can reflectthe entire light transmitted through the core layer at the connectingportion connected to the core layer.

The connecting portion between the support and the core layer may bepositioned at any apposition on the surface, opposing the substrate, ofthe core layer including, for example, a central portion in the widthdirection. Also, the connecting portion between the support and the corelayer may be at any position on the end surface of the core layer in thewidth direction. Also, the connecting portion between the support andthe core layer may be intermittently arranged along the longitudinaldirection of the core layer. This configuration enlarges the outersurface that does not contact the support in a portion of the core layerin the longitudinal direction, and thus can enlarge an interaction areabetween the evanescent wave and the target gas or the target liquid.Further, the connecting portion between the support and the core layermay have a shape extending in the longitudinal direction of the corelayer as it is positioned closer to the central position of the corelayer away from the edge portion of the core layer in the widthdirection. By virtue of this shape, the surrounding conditions of thecore layer gradually change along the longitudinal direction of the corelayer from an area that does not include the support to an area thatincludes the support (or vice versa). Thus, a sudden change of thesurrounding conditions for the light transmitted through the core layercan be avoided, and a scattering loss of the light transmitted throughthe core layer can be reduced.

A method for forming the support is, for example, etching a BOX (BuriedOxide) layer in the SOI (Silicon On Insulator) substrate, whereby astructure in which the core layer (Si layer) is supported by the BOXlayer with respect to the substrate (Si layer) can be formed.

Protrusion

The protrusions are intermittently arranged in the longitudinaldirection of the core layer in the space between the substrate and thecore layer. The protrusions protrude toward the core layer from thesubstrate.

The protrusions may be formed from the same material as the support.This facilitates the formation of the protrusions arranged in an areaoverlapping with the support in the thickness direction of the corelayer by employing the lithography technology and the etchingtechnology. Also, the protrusions may have a mountain-like shape. Thus,in a configuration in which the core layer has, for example, arectangular shape in a plane perpendicular to the longitudinaldirection, the bottom surface of the core layer and the surface of theprotrusions having the mountain-like shape are not in parallel with eachother, and sticking between the core layer and the protrusion isunlikely to occur. When the protrusions do not have a mountain-likeshape, a similar effect can be obtained by virtue of that the bottomsurface of the core layer and the surface of the protrusions opposingthe core layer are not in parallel with each other. Note that themountain-like shape of the protrusions is shapes of the protrusionsviewed from any direction perpendicular to the thickness direction ofthe core layer, e.g., the longitudinal direction of the core layer.Although each of the protrusions may have an asymmetrical shape in thewidth direction of the core layer, each of the protrusions preferablyhas a symmetrical shape. The symmetrical shape in the width direction ofthe core layer means a line-symmetrical shape with a straight lineperpendicular to the width direction serving as the axis of symmetry.Thus, in a configuration in which the core layer has a symmetrical shapein the width direction and each of the protrusions also has asymmetrical shape in the width direction, when light transmits to thecore layer in an area that includes the protrusion immediately below thecore layer from the care layer in an area that does not include theprotrusion immediately below the core layer, mode conversion of light issuppressed, and transmission loss due to the mode conversion is reduced.The maximum height of the protrusion may be 1/20 or more of the distancebetween the core layer and the substrate. This configuration cansuppress the occurrence of sticking of the core layer. The maximumheight of the protrusion may be 100 nm or more. This configuration canfurther suppress the occurrence of sticking of the core layer. Also, inthe longitudinal direction of the core layer, a given protrusion mayinclude, therearound, an area that does not include the protrusion andthe support, that is, an area between two protrusions adjacent to eachother in the longitudinal direction or an area between the protrusionand the support adjacent to each other in the longitudinal direction. Alongitudinal length of the area that does not include the protrusion andthe support may be equal to or smaller than:

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack & \; \\{\mspace{349mu}{\frac{1}{2}{\left( \frac{{Eh}^{2}t^{3}}{\alpha} \right)^{\frac{1}{4}}.}}} & \;\end{matrix}$Here, E is a Young's modulus [N/m²] of the core layer, h is a floatingdistance [m] between the core layer and substrate, t is a thickness [m]of core layer, and α is an experimental proportional parameter45.2345×10⁻⁶ [N/m]. Also, the longitudinal length of the area that doesnot include the protrusion and the support may be 71 μm or less. In thisway, the area of the core layer in the longitudinal direction that doesnot include the support and the protrusion can be equal to or smallerthan:

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack & \; \\{\mspace{349mu}{{\frac{1}{2}\left( \frac{{Eh}^{2}t^{3}}{\alpha} \right)^{\frac{1}{4}}},}} & \;\end{matrix}$or 71 μm or less, whereby the occurrence of sticking in the opticalwaveguide can be suppressed. Further, a position of the maximum heightof the protrusion may be deviated from the central position of the corelayer in the width direction. However, the protrusion does not need tohave the maximum height at a position deviated from the central positionof the core layer in the width direction over the entire region of thecore layer in the longitudinal direction. For example, the protrusionmay have the maximum height at a position deviated from the centralposition of the core layer in the width direction in at least an area ofthe core layer in the longitudinal direction and have a maximum heightat the central position of the core layer in the width direction inanother area.Optical Concentration Measuring Apparatus

An optical concentration measuring apparatus according to the presentembodiment includes the optical waveguide according to the presentembodiment, a light source capable of irradiating light into the corelayer, and a detector capable of detecting light transmitted through thecore layer.

Each constituent element of the optical concentration measuringapparatus will be described below with specific examples.

Light Source

The light source may be any light source capable of irradiating lightinto the core layer. When infrared radiation is used for the measurementof a gas, an incandescent bulb, a ceramic heater, a MEMS(Microelectromechanical systems) heater, an infrared LED (Light-EmittingDiode), or the like can be used as the light source. The light sourcemay be arranged in any manner that can be optically connected to theoptical waveguide. For example, the light source may be arrangedadjacent to the optical waveguide in the same constituent element, ormay be arranged at a certain distance from the optical waveguide as adifferent constituent element. When ultraviolet rays are used for themeasurement of a gas, a mercury lamp, an ultraviolet LED, or the likecan be used as the light source. When x-rays are used for themeasurement of a gas, an electron beam, an electron laser, or the likecan be used as the light source.

The light transmitted through the core layer of the optical waveguideprovided in the optical concentration measuring apparatus may beinfrared radiation serving as an analog signal. Here, the infraredradiation serving as the analog signal is not for determination on anenergy change of light between zero (low level) and 1 (high level), andrepresents a signal for an amount of the energy change of light. Thus,the optical concentration measuring apparatus can be used for sensorsand analysis equipment. In this case, the wavelength, in vacuum, of theinfrared radiation may be 2 μm to 12 μm. This is the wavelength bandabsorbed by gasses (CO₂, CO, NO, N₂O, SO₂, CH₄, H₂O, C₂H₆O, and thelike) that are typically floating in the environment. Accordingly, theoptical concentration measuring apparatus according to the presentembodiment can be used as a gas sensor.

Detector

The detector may be any detector capable of receiving light transmittedthrough the core layer of the optical waveguide. When infrared radiationis used for the measurement of a gas, a thermal infrared sensor such asa pyroelectric sensor, a thermopile, or a bolometer, or a quantuminfrared sensor such as a diode or a phototransistor may be used as thedetector. When ultraviolet rays are used for the measurement of a gas, aquantum ultraviolet sensor such as a diode or a phototransistor may beused as the detector. When x-rays are used for the measurement of a gas,various types of a semiconductor sensor may be used as the detector.

The optical waveguide and an optical concentration measuring apparatusaccording to the present embodiment will be described with reference toFIG. 20 to FIG. 36.

FIG. 20 is a diagram illustrating a schematic configuration of anoptical concentration measuring apparatus 6 according to the presentembodiment and is also a conceptual drawing of the ATR method using theoptical waveguide 10 according to the present embodiment.

As illustrated in FIG. 1, the optical concentration measuring apparatus6 is installed and used in the external space 2 that includes a gaswhose concentration or the like is to be detected. The opticalconcentration measuring apparatus 6 includes an optical waveguide 60according to the present embodiment, a light source 20 capable ofirradiating light (infrared radiation IR in the present embodiment) intoa core layer 11 provided in the optical waveguide 60, and aphotodetector (an example of a detector) 40 capable of receiving theinfrared radiation IR transmitted through the core layer 11.

The optical waveguide 60 includes a substrate 15, the core layer 11 thatcan transmit the infrared radiation IR (an example light), supports 17each of which intermittently connects at least a portion of thesubstrate 15 and a portion of the core layer 11 together and thussupports the core layer 11 with respect to the substrate 15, andprotrusions 68 that protrude toward the core layer 11 from the substrate15. The core layer 11 and the substrate 15 are made of silicon (Si), andthe supports 17 and the protrusions 18 are made of silicon dioxide(SiO₂).

The substrate 15 has, for example, a plate-like shape. The core layer 11is, for example, a rectangular parallelepiped in part. The core layer 11includes a curved portion in which the rectangular parallelepiped iscurved about an axis in a direction perpendicular to the principlesurface of the substrate 15, as illustrated in FIG. 21.

As illustrated in FIG. 20, the optical waveguide 60 includes a gratingcoupler 118 formed at one end of the core layer 11 in the longitudinaldirection and a grating coupler 119 formed at the other end of the corelayer 11 in the longitudinal direction. The grating coupler 118 isarranged in the irradiating direction of the light source 20. In thepresent embodiment, the optical waveguide 60 is arranged such that thestacking direction therein is parallel to the vertical direction, andthe principle surface of the substrate 15 is directed in the verticallydownward direction. The irradiating direction of the light source 20 isvertically downward from the light source 20 in a state in which theoptical waveguide 10 is installed in the above manner. The gratingcoupler 118 couples the infrared radiation IR irradiated by the lightsource 20 with the infrared radiation IR transmitted through the corelayer 11. The grating coupler 119 is arranged in a direction facing thephotodetector 40. Note that “the direction facing the photodetector 40”is a direction vertically downward from the photodetector 40 in a statein which the optical waveguide 10 is installed in the above manner. Thegrating coupler 119 is configured to extract the infrared radiation IRtransmitted through the core layer 11 and pass the extracted infraredradiation IR to the photodetector 40.

FIG. 22 is a cross-sectional end view of the optical waveguide 60 takenfrom line E-E in FIG. 20, and FIG. 23 is a cross-sectional end view ofthe optical waveguide 60 taken from line F-F in FIG. 20. FIG. 24 is across-sectional end view of the optical waveguide 60 taken from line G-Gin FIG. 20.

As illustrated in FIG. 20, FIG. 22, FIG. 23, and FIG. 24, the opticalwaveguide 60 includes gaps 13 between the core layer 11 and thesubstrate 15 without having a predetermined layer such as a clad layer,except for the areas that include the supports 17.

As illustrated in FIG. 22, a connecting portion 171 of the support 17connected to the core layer 11 overlaps with the central position cp ofthe core layer 11 in the width direction. As illustrated in FIG. 20,also, the connecting portions 171 of the supports 17 are intermittentlyarranged along the longitudinal direction. As illustrated in FIG. 20 andFIG. 21, further, the protrusions 68 are intermittently arranged in thespace between the substrate 15 and the core layer 11 in the longitudinaldirection of the core layer 11.

In the space, the protrusions 68 are arranged at positions differentfrom the positions of the supports 17 in the longitudinal direction ofthe core layer 11. As illustrated in FIG. 23, the protrusion 68 isarranged at a position overlapping with the central position cp of thecore layer 11 in the width direction in the space. The protrusion 68 hasa mountain-like shape as viewed from the longitudinal direction of thecore layer 11. The protrusion 68 has a symmetrical shape in the widthdirection of the core layer 11. A maximum height of the protrusion 68 is1/20 or more of the gap gp between the core layer 11 and the substrate15, which is 100 nm or more, in particular. The protrusion 68 is alsoarranged immediately below the curved portion of the core layer 11 asillustrated in FIG. 21. In the longitudinal direction of the core layer11, a length of an area that does not include the support 17 and theprotrusion 68 in the longitudinal direction of the core layer 11, thatis, an area between two protrusions 68 adjacent to each other in thelongitudinal direction of the core layer 11 or an area between aprotrusion 68 and a support 17 adjacent to each other in thelongitudinal direction of the core layer 11 is equal to or smaller than:

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 3} \right\rbrack & \; \\{\mspace{346mu}{{\frac{1}{2}\left( \frac{{Eh}^{2}t^{3}}{\alpha} \right)^{\frac{1}{4}}},}} & \;\end{matrix}$which is equal to or smaller than 71 μm, in particular. Here, E is theYoung's modulus [N/m²] of the core layer, h is the gap gp [m] betweenthe core layer 11 and substrate 15, t is the thickness [m] of core layer11, and a is the experimental proportional parameter 45.2345×10⁻⁶ [N/m].

The shape of the protrusion 68 and the positions of the protrusions 68with respect to the width direction of the core layer 11 can be observedin the following manner. To observe a cross-sectional shape of theprotrusion 68, a SEM (scanning electron microscope) is used to observe aplane perpendicular to the longitudinal direction. To observe a relativeposition between the protrusion 68 and the core layer 11 with respect tothe width direction, a damaged layer is formed by emitting an FIB(Focused Ion Beam) from a side of the core layer 11 of the opticalwaveguide 60 such that a shadow of the core layer 11 is transferred ontothe substrate. Then, the core layer 11 is peeled off, and the shadow,transferred onto the peeled surface, of the core layer 11 and theprotrusion 68 are observed using the SEM. In particular, the FIB isemitted in the direction perpendicular to the principle surface of thesubstrate 15 at a portion where the core layer 11 is floating. Thus, theshadow of the core layer 11 is transferred onto the substrate 15 by thedamaged layer generated by the FIB. The transferred shadow of the corelayer 11 indicates the position of the core layer 11 in a plane parallelto the principle surface of the substrate 15. Thus, by comparing thetransferred shadow of the core layer 11 and the position of theprotrusion 68, the relative position between the protrusion 68 and thecore layer 11 can be determined.

Here, an effect of the optical waveguide 60 according to the presentembodiment will be described by comparing with an optical waveguide 60′that includes a protrusion 18′ arranged over the entire region in thelongitudinal direction, as illustrated in FIG. 37.

A sensor that employs the ATR method is often set to transmit light in asingle mode within the core layer. In the optical concentrationmeasuring apparatus 6 according to the present embodiment, light(infrared radiation) is transmitted in single-mode by way of examplewithin the core layer 11 of the optical waveguide 60. Note that, whenlight is transmitted in a multi-mode, the light is transmitted in thelongitudinal direction of the core layer, whereby the effect of thepresent disclosure can be obtained. As illustrated in FIG. 20, wheninfrared radiation IR is transmitted in the single mode within the corelayer 11, the intensity of the infrared radiation decreases with adistance from the central portion of the core layer 11, and anevanescent wave EW extends from the vicinity of the core layer 11. Notethat the evanescent wave EW of the infrared radiation IR transmittedthrough the core layer 11′ of the optical waveguide 60′, in which theprotrusions 68′ are arranged over the entire region in the longitudinaldirection between the supports 17′ adjacent to each other as illustratedin FIG. 37, has the same distribution as that of the optical waveguide60 of the present embodiment.

In a sensor that employs the ATR method, the sensitivity of the sensoris improved by expanding the region of interaction between theevanescent wave extending from the core layer and a substance to bemeasured (that is, by increasing an exposed portion of the core layer)and suppressing optical absorption by materials other than the substanceto be measured (that is, absorption of light by the support or thelike). Arranging the protrusions 68′ over the entire region in thelongitudinal direction between the supports 17′ adjacent to each otheras illustrated in FIG. 37 for the purpose of suppressing occurrence ofsticking can be considered as the simplest configuration.

However, in the optical waveguide 60′, the evanescent wave EW isabsorbed by the material forming the protrusion 68′ over the entireregion in the longitudinal direction in the gap 13 between the corelayer 11′ and the substrate 15′. As a result, the sensitivity of thesensor using the optical waveguide 60′ is deteriorated.

As illustrated in FIG. 20, the optical waveguide 60 according to thepresent embodiment has a structure for suppressing sticking caused bythe protrusions 68 disposed in the gap between the core layer 11 and thesubstrate 15, in a manner similar to the optical waveguide 60′. Asillustrated in FIG. 20, however, the protrusions 68 are intermittentlyarranged in the longitudinal direction of the core layer 11. Thus, theevanescent wave EW extending from the core layer 11 to its periphery isnot absorbed in the areas that do not include the protrusions 68,whereby the amount of light (infrared radiation) absorbed by theprotrusions 68 while transmitting from the grating coupler 118 to thegrating coupler 119 is reduced. In this way, the optical waveguide 60according to the present embodiment can reduce the optical absorption ofthe evanescent wave EW by the protrusions 68, as compared to the opticalwaveguide 60′ in which the protrusions 68′ are continuously arranged inthe longitudinal direction of the core layer 11′ in an ordinary manner.

Each of the protrusions 68 has a mountain-like shape as viewed from thelongitudinal direction of the core layer 11. Thus, in a configuration ofthe present embodiment in which the core layer 11 has a rectangularshape within the plane perpendicular to the longitudinal direction, thebottom surface of the core layer 11 and the surfaces of the protrusions68 do not become parallel with each other, whereby sticking between thecore layer and the protrusions is unlikely to occur. Also, each of theprotrusions 68 has a symmetrical shape in the width direction of thecore layer 11. Thus, in the present embodiment in which the core layer11 has a symmetrical shape in the width direction, by virtue of thesymmetrical shape of the protrusions 68 in the width direction, modeconversion of the light is unlikely to occur while the light transmitsfrom a core layer in an area that does not include a protrusion 68immediately below the core layer to a core layer in an area thatincludes a protrusion 68 immediately below the core layer 11, andtransmission loss due to the mode conversion is reduced. The maximumheight of the protrusion 68 is 1/20 or more of the gap gp between thecore layer 11 and the substrate 15, which is 100 nm or more, inparticular. This configuration can further suppress the occurrence ofsticking of the core layer 11. Also, for the purpose of miniaturizationof the optical waveguide 60 in its entirety, the core layer 11 includesthe curved portion, immediately under which the protrusion 68 isarranged. Thus, the optical waveguide 60 can improve the durabilityagainst sticking of the curved portion of the core layer 11 where thedurability is otherwise lower than that of the linear portion. In thelongitudinal direction of the core layer 11, the length of the area thatdoes not include the support 17 and the protrusion 68 is equal to orsmaller than:

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 4} \right\rbrack & \; \\{\mspace{346mu}{{\frac{1}{2}\left( \frac{{Eh}^{2}t^{3}}{\alpha} \right)^{\frac{1}{4}}},}} & \;\end{matrix}$or 71 μm or more, in particular.

Here, E is a Young's modulus [N/m²] of the core layer, h is the gap gp[m] between the core layer 11 and substrate 15, t is the thickness [m]of core layer 11, and α is the experimental proportional parameter45.2345×10⁻⁶ [N/m]. In this way, the area in the longitudinal directionof the core layer 11 that does not include the support 17 and theprotrusion 68 can be regulated to be equal to or smaller than:

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 5} \right\rbrack & \; \\{\mspace{346mu}{{\frac{1}{2}\left( \frac{{Eh}^{2}t^{3}}{\alpha} \right)^{\frac{1}{4}}},}} & \;\end{matrix}$or 71 μm or less, whereby the occurrence of sticking in the opticalwaveguide 60 can be suppressed.

Here, in the longitudinal direction of the core layer 11 of the opticalwaveguide 60 according to the present embodiment, the area in thelongitudinal direction of the core layer 11 that does not include thesupport 17 and the protrusion 68 will be described in more detail.

In the present disclosure, the protrusions 68 are intermittentlyarranged in the longitudinal direction below the core layer 11. Such anintermittent arrangement of the protrusions 68 corresponds to theconfiguration of the optical waveguide 60 that has the configuration inwhich the core layer 11 having a floating structure with respect to thesubstrate 15 includes an area in which a given protrusion 68 does nothave another protrusion 68 or the support 17 provided therearound. Here,such an area in which a given protrusion 68 does not include anotherprotrusion 68 or the support 17 therearound may be classified into thefollowing two types:

I: Area from a given protrusion 68 to the next protrusion 68

II: Area from a given protrusion 68 to a support 17.

Here, because the support 17 more effectively suppress sticking than theprotrusion 68, a distance of II can be longer than a distance of I.Thus, in the optical waveguide 60 that includes the protrusions 68intermittently arranged, a maximum value of the distance which canprevent sticking without a protrusion 68 can be estimated from thedistance II. That is, in the optical waveguide 60 having theconfiguration in which the core layer 11 is supported by two supports 17and floats with respect to the substrate 15, by examining the distancebetween the two supports 17 with respect to a region in which stickingoccurs in a state in which a protrusion 68 is not provided between thetwo supports 17, a maximum distance around the protrusion 68 that cansuppress sticking without another protrusion 68 or a support 17 can beestimated.

FIG. 38 is a microscopy image (a plan view) of the optical waveguide 60observed when the optical waveguide 60 was produced without forming theprotrusion 68 below the core layer 11, the core layer 11 (silicon)having the thickness of 220 nm, the width of 2 μm and the floatingdistance between the core layer 11 and the substrate 15 (i.e., theheight of the support 17 from the surface of the substrate 15) of 3 μm,and the distance between supports 17 being 150 μm. In FIG. 38, the corelayer 11 is in contact with the substrate 15 due to sticking around thecentral portion of the core layer 11, ranging 45 μm, in the distance of150 μm between the supports 17 (while the remainder of the core layer 11is floating). As illustrated in FIG. 38, in the rhombus-shaped areas ofthe core layer 11 on the left and right sides, the support 17 isprovided between the core layer 11 and the substrate 15, and the narrowarea of the core layer 11 between the rhombus-shaped areas includeneither the support 17 nor the protrusion 68. From this measurementresult, the maximum distance 1 p between the protrusion 68 and thesupport 17 capable of suppressing sticking can be determined as anequation (1):

$\begin{matrix}{\left\lbrack {{Formula}\mspace{14mu} 6} \right\rbrack\mspace{625mu}} & \; \\{l_{p} = {\frac{1}{2}{\left( \frac{{Eh}^{2}t^{3}}{\alpha} \right)^{\frac{1}{4}}.}}} & (1)\end{matrix}$

Here, E is Young's modulus [N/m²] of a beam (i.e., core layer 11), histhe gap gp [m] between the core layer 11 and substrate 15, t is athickness [m] of the beam (the core layer 11), and α is the experimentalproportional parameter 45.2345×10⁻⁶ [N/m].

In order to obtain the equation (1), the optical waveguide 60 describedabove is assumed as a beam which is rectangular shape in cross-sectionand is fixed at both ends of it, and a model in which the Laplacepressure caused by the surface tension of chemical solution accumulatedbetween the core layer 11 and the substrate 15 during the manufacturingprocess is a uniform load is applied, then a deflection amount iscalculated using equations (2), (3) and (4).

$\begin{matrix}{\left\lbrack {{Formula}\mspace{14mu} 7} \right\rbrack\mspace{625mu}} & \; \\{{v(x)} = {\frac{{pl}^{2}x^{2}}{24{EI}}\left( {1 - \frac{2x}{l} + \frac{x^{2}}{l^{2}}} \right)}} & (2) \\{I = \frac{{wt}^{3}}{12}} & (3) \\{p = \frac{2\gamma\; w\;\cos\;\theta}{h}} & (4)\end{matrix}$

In the equations (2) to (4), v(x) is the deflection amount at a distancex from one end, p is a load per unit length, l is a total length of thebeam (i.e., the floating portion of the core layer 11), I is thegeometrical moment of inertia, w is a width of the beam, γ is thesurface tension, and θ is a contact angle. In the optical waveguide 60having the size described above with reference to FIG. 38 (i.e., thethickness of the core layer 11 is 220 nm, the width of the core layer 11is 2 μm, the floating distance between the core layer 11 and thesubstrate 15 is 3 μm, and the distance between supports is 150 μm), whenthe Young's modulus of the core layer 11 corresponds to the Young'smodulus of single crystal silicon (193 GPa), the load p can bedetermined such that the length of the x region in which the deflectionamount takes a value larger than the floating distance h (i.e., 3 μm),which means the distance in a state contacting the substrate 15, becomes45 μm.

On the other hand, because the maximum deflection amount v_(max)corresponds to the deflection at the central portion of the beam (i.e.,x=½), v_(max) can be calculated as equations (5) and (6) by using theequations (2), (3) and (4).

$\begin{matrix}{\left\lbrack {{Formula}\mspace{14mu} 8} \right\rbrack\mspace{625mu}} & \; \\{v_{\max} \propto \frac{l^{4}}{{Eht}^{3}}} & (5) \\{{\therefore v_{\max}} = {\alpha\;\frac{l^{4}}{{Eht}^{3}}}} & (6)\end{matrix}$

At this time, the experimental proportional parameter α can be obtainedby using the load p calculated from the measuring result.

Incidentally, the protrusion 68 can suppress sticking when provided at acontact position between the core layer 11 and the substrate 15. Thus,when the protrusion 68 is positioned at a position half the entirelength of the floating portion of the core layer 11 in a condition thatthe core layer 11 contacts the substrate 15 at just one point (i.e.,when the maximum deflection amount v_(max) and the floating distance hbetween the core layer 11 and the substrate 15 are equal to each other),sticking can be suppressed. From this condition, l_(p) can be calculatedas equation (9), via equations (7), (8).

$\begin{matrix}{\left\lbrack {{Formula}\mspace{14mu} 9} \right\rbrack\mspace{625mu}} & \; \\{v_{\max} = {{\alpha\;\frac{l^{4}}{{Eht}^{3}}} = h}} & (7) \\{l = \left( \frac{{Eh}^{2}t^{3}}{\alpha} \right)^{\frac{1}{4}}} & (8) \\{{\therefore l_{p}} = {\frac{1}{2}\left( \frac{{Eh}^{2}t^{3}}{\alpha} \right)^{\frac{1}{4}}}} & (9)\end{matrix}$

From the equation (9), in particular, in a configuration in which theoptical waveguide 60 having the core layer 11 made of silicon has thecross-sectional dimensions described with reference to FIG. 38 (i.e.,the thickness of the core layer 11 is 220 nm and the floating distancebetween the core layer 11 and the substrate 15 is 3 μm), sticking can besuppressed by arranging the protrusion 68 at a position of 71 μm or lessin the longitudinal direction from the support 17. That is, in theoptical waveguide 60 that includes the protrusions 68 intermittentlyarranged, the maximum value of the distance at which sticking does notoccur without the protrusions 68 is 71 μm.

Next, a method for manufacturing the optical waveguide 60 according tothe present embodiment will be described with reference to FIG. 25 toFIG. 36 using FIG. 20 to FIG. 24. FIG. 25 illustrates a manufacturingprocess plan view of the optical waveguide 60. FIG. 26 illustrates amanufacturing process end view of the optical waveguide 60 taken fromlines H-H, I-I, and J-J in FIG. 25. FIG. 27 illustrates a manufacturingprocess plan view of the optical waveguide 60. FIG. 28 illustrates amanufacturing process end view of the optical waveguide 60 taken fromlines H-H, I-I, and J-J in FIG. 27. FIG. 29 illustrates a manufacturingprocess plan view of the optical waveguide 60. FIG. 30 is amanufacturing process end view of the optical waveguide 60 taken fromline I-I in FIG. 29. FIG. 31 illustrates a manufacturing process endview of the optical waveguide 60 taken from line J-J in FIG. 29. Notethat the manufacturing process end view of the optical waveguide 60taken from line H-H line in FIG. 29 does not include the mask pattern M3and the mask pattern M4 and thus has the same configuration as themanufacturing process end view illustrated in FIG. 28. FIG. 32illustrates a manufacturing process plan view of the optical waveguide60. FIG. 33 illustrates a manufacturing process end view of the opticalwaveguide 60 taken from line H-H in FIG. 32. FIG. 34 illustrates amanufacturing process end view of the optical waveguide 60 taken fromline I-I in FIG. 32. FIG. 35 illustrates a manufacturing process endview of the optical waveguide 60 taken from line J-J in FIG. 32. FIG. 36illustrates a manufacturing process end view in which the opticalwaveguide main portion of a variation of the present embodiment is takenfrom the same position as the line I-I in the optical waveguide mainportion 60 a illustrated in FIG. 32.

First, a SiO₂ film is formed on one or both of a support substrate 15 athat is made of silicon and eventually formed as the substrate 15, andan active substrate 11 a that is made of silicon and eventually formedas the core layer 11. Next, the support substrate 15 a and the activesubstrate 11 a are stuck together with the SiO₂ film therebetween andsubjected to heat treatment to be joined together. Then, the filmthickness of the active substrate 11 a is adjusted to a predeterminedthickness by grinding, polishing, or the like. Thus, an SOI substrate100 having a “silicon-insulating layer-silicon” structure that includesthe support substrate 15 a, a BOX layer 17 a formed on the supportsubstrate 15 a, and the active substrate 11 a formed on the BOX layer 17a is formed as illustrated in FIG. 25 and FIG. 26.

Next, lithography and etching are performed on the SOI substrate 100 toetch the active substrate 11 a and form the core layer 11 that has arectangular cross-section and extends in the longitudinal direction.Thus, the optical waveguide main portion 60 a that includes a supportsubstrate 15 a having a plate-like shape, the BOX layer 17 a having aplate-like shape and formed on the support substrate 15 a, and the corelayer 11 formed as a rectangular prism on a portion of the BOX layer 17a is formed as illustrated in FIG. 27 and FIG. 28.

Next, the mask patterns M3 and mask patterns M4 that cover portions ofthe core layer 11 and portions of the BOX layer 17 a are intermittentlyformed in the longitudinal direction of the core layer 11 as illustratedin FIG. 29. The mask patterns M3 are arranged in a symmetrical mannerwith respect to the central position of the core layer 11 in the widthdirection as illustrated in FIG. 29 and FIG. 30. The mask patterns M4are arranged in a symmetrical manner with respect to the center of thecore layer 11 in the width direction and wider than the mask patterns M3as illustrated in FIG. 29 and FIG. 31. The mask patterns M3 and the maskpatterns M4 may be formed from photoresist or hard masks such as siliconnitride films. Also, the mask patterns M3 and the mask patterns M4 maybe an integral mask layer (i.e., one mask layer) or individual masklayers.

Next, a portion of the BOX layer 17 a of the optical waveguide mainportion 60 a is removed by performing wet etching or the like using themask patterns M3 and the mask patterns M4 as masks. Thus, in the areasof the core layer 11 that do not have the mask patterns M3 or the maskpatterns M4 in the longitudinal direction, the support 17 and theprotrusion 68 are not formed and the gap 13 is formed as illustrated inFIG. 32 and FIG. 33. In each area of the core layer 11 that has the maskM3 in the longitudinal direction, the protrusion 68 is formed at thecentral position of the core layer 11 in the width direction asillustrated in FIG. 32 and FIG. 34. In each area of the core layer 11that has the mask M4 in the longitudinal direction, the support 17 isformed at the central position of the core layer 11 in the widthdirection as illustrated in FIG. 32 and FIG. 35. Although a portion ofthe BOX layer 17 a other than the portion forming the support 17 and theprotrusion 18 is removed in FIG. 32 to FIG. 35, this portion of the BOXlayer 17 a does not need to be completely removed. As illustrated in avariation of the present embodiment referring to in FIG. 36, the thinfilm 19 covering the support substrate 15 a may be formed from theremainder of the BOX layer 17 a. At this time, the maximum height of theprotrusion 68 corresponds to the height of the peak of the mountain-likeshape of the protrusion 68 from a lowest position of the surface of thethin film 19 in the region immediately below the core layer 11.

Next, the mask patterns M3 and the mask patterns M4 are etched. Althoughthe formation of the grating couplers is omitted in the presentembodiment, when the grating couplers 118 and 119 as illustrated in FIG.20 are formed, the grating couplers 118 and 119 may be formedsimultaneously with, before, or after the formation of the core layer 11illustrated in FIG. 27, followed by the formation of the mask patternsM3 and M4 illustrated in FIG. 29. The configuration illustrated in FIG.20 is obtained by forming the grating coupler 118 in a slit-like shapeat one end portion of the core layer 11 in the longitudinal directionand forming the grating coupler 119 having a slit-like shape at theother end portion of the core layer 11 in the longitudinal direction.

Then, the support substrate 15 a is cut in a predetermined region toseparate the optical waveguide main portion 60 a. Thus, the opticalwaveguide 60 in which the protrusions 68 are intermittently provided inthe longitudinal direction of the core layer 11 is produced (see FIG. 20to FIG. 24).

As illustrated in FIG. 20, further, the optical concentration measuringapparatus 6 is produced by installing the light source 20 in such amanner as to be able to irradiate the infrared radiation IR into thegrating coupler 118 of the optical waveguide 60 and by arranging thephotodetector 40 in such a manner as to be able to receive the infraredradiation IR exit from the grating coupler 119 of the optical waveguide60.

As described above, the optical waveguide 60 has a configuration inwhich the protrusions 68 are intermittently arranged in the longitudinaldirection of the core layer 11. Thus, the optical waveguide 60 cansuppress the occurrence of sticking of the core layer 11 and preventdeterioration of the detecting characteristics for detecting a substanceto be measured MO caused by the protrusions 68.

According to the present embodiment, as described above, the opticalwaveguide 60 and the optical concentration measuring apparatus 6 thatinclude the protrusions 68 which suppress the occurrence of sticking ofthe core layer 11 and prevent deterioration of the sensitivity of thesensor can be provided.

Further, the optical waveguide 60 according to the present embodimentcan reduce the amount of the evanescent wave EW absorbed by theprotrusions 68. Thus, the optical waveguide 60 according to the presentembodiment can detect the substance to be measured MO with highsensitivity in a stable manner in various specification modes.

APPENDIX

[1] An optical waveguide used in an optical concentration measuringapparatus for measuring concentration of a target gas or a target liquidto be measured, the optical waveguide includes: a substrate; a corelayer that can transmit light and extends along a longitudinaldirection; a support that connects at least a portion of the substrateand a portion of the core layer together and supports the core layerwith respect to the substrate; and protrusions that are intermittentlyarranged in the longitudinal direction in a space between the substrateand the core layer and protrude toward the core layer from thesubstrate.

[2] The optical waveguide according to [1],

wherein the support is made of a material having a smaller refractiveindex than the core layer.

[3] The optical waveguide according to [1],

wherein the protrusions are made the same material as the support.

[4] The optical waveguide according to [1],

wherein each of the protrusions has a mountain-like shape.

[5] The optical waveguide according to [1],

wherein each of the protrusions has a symmetrical shape in a widthdirection of the core layer.

[6] The optical waveguide according to [1],

wherein each of the protrusions has an asymmetrical shape in a widthdirection of the core layer.

[7] The optical waveguide according to [1],

wherein a maximum height of the protrusions is equal to or more than1/20 of a gap between the core layer and the substrate.

[8] The optical waveguide according to [1],

wherein a maximum height of the protrusions is 100 nm or more.

[9] The optical waveguide according to [1],

wherein the core layer extending in the longitudinal direction includesa curved portion, and the protrusions are arranged immediately below thecurved portion.

[10] The optical waveguide according to [1],

wherein there is an area that does not include the protrusion and thesupport around a given protrusion in the longitudinal direction, and alongitudinal length of the area is equal to or smaller than:

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 10} \right\rbrack & \; \\{\mspace{349mu}{{\frac{1}{2}\left( \frac{{Eh}^{2}t^{3}}{\alpha} \right)^{\frac{1}{4}}},}} & \;\end{matrix}$where E is Young's modulus [N/m²] of the core layer, h is a floatingdistance [m] between the core layer and substrate, t is a thickness [m]of core layer, and α is an experimental proportional parameter45.2345×10⁻⁶ [N/m].

[11] The optical waveguide according to [1],

wherein there is an area that does not include the protrusion and thesupport around a given protrusion in the longitudinal direction, and alongitudinal length of the area is 71 μm or less.

[12] The optical waveguide according to [1],

wherein maximum height positions of the protrusions in the widthdirection of the core layer are deviated from a central position of thecore layer.

[13] The optical waveguide according to [1],

wherein at least a portion of the core layer is exposed or covered witha thin film.

[14] The optical waveguide according to [1],

wherein the light transmitted through the core layer is infraredradiation serving as an analogue signal.

[15] An optical concentration measuring apparatus comprising:

the optical waveguide according to [1];

a light source capable of irradiating light into the core layer; and

a detector capable of receiving light transmitted through the corelayer.

[16] The optical concentration measuring apparatus according to [15],

wherein the light source is configured to irradiate infrared radiationhaving a wavelength, in vacuum, of 2 μm to 12 μm into the core layer.

The invention claimed is:
 1. An optical waveguide used in an opticalconcentration measuring apparatus for measuring concentration of a gasor a liquid to be measured, the optical waveguide comprising: asubstrate; a core layer that can transmit light and extends along alongitudinal direction; a support that connects at least a portion ofthe substrate and a portion of the core layer together and supports thecore layer with respect to the substrate; and a protrusion that isarranged at a position different from a position of the support in aspace between the substrate and the core layer and protrudes toward thecore layer from the substrate in such a manner as to have a maximumheight at a position deviated from a central position of the core layerin a width direction.
 2. The optical waveguide according to claim 1,wherein the support is made of a material having a smaller refractiveindex than the core layer.
 3. The optical waveguide according to claim1, wherein the protrusion is made of the same material as that of thesupport.
 4. The optical waveguide according to claim 1, wherein theprotrusion has a mountain-like shape.
 5. The optical waveguide accordingto claim 1, wherein the protrusion has a symmetrical shape in the widthdirection of the core layer.
 6. The optical waveguide according to claim1, wherein the protrusion has an asymmetrical shape in the widthdirection of the core layer.
 7. The optical waveguide according to claim1, wherein the protrusion has a maximum height at a position deviatedfrom the central position of the core layer in the width direction by 3%or more of a width of the core layer.
 8. The optical waveguide accordingto claim 1, wherein the protrusion has a maximum height at a positiondeviated from the central position of the core layer in the widthdirection by 100 nm or more.
 9. The optical waveguide according to claim1, wherein a maximum height of the protrusion is 1/20 or more of a gapbetween the core layer and the substrate.
 10. The optical waveguideaccording to claim 1, wherein a maximum height of the protrusion is 100nm or more.
 11. The optical waveguide according to claim 1, wherein thecore layer extending along the longitudinal direction includes a curvedportion, and the protrusion is arranged immediately below the curvedportion.
 12. The optical waveguide according to claim 11, wherein theprotrusion is arranged immediately below the core layer at a positionclose to an inner periphery of the core layer in the curved portion. 13.The optical waveguide according to claim 11, wherein the protrusion isarranged immediately below the core layer at a position close to anouter periphery of the core layer in the curved portion.
 14. The opticalwaveguide according to claim 1, wherein a plurality of protrusions areintermittently arranged along the longitudinal direction of the corelayer.
 15. The optical waveguide according to claim 1, wherein at leasta portion of the core layer is exposed or covered with a thin film. 16.The optical waveguide according to claim 1, wherein light transmittedthrough the core layer is infrared radiation serving as an analoguesignal.
 17. An optical concentration measuring apparatus comprising: theoptical waveguide according to claim 1; a light source capable ofirradiating light into the core layer; and a detection unit capable ofreceiving light transmitted through the core layer.
 18. The opticalconcentration measuring apparatus according to claim 17, wherein thelight source irradiates infrared radiation of wavelength, in vacuum, of2 μm to 12 μm into the core layer.